The present invention relates to an antilock brake control method for motor vehicles.
In hydraulic braking systems of the prior art for motor vehicles, there have been proposed and demonstrated various types of antilock brake control devices in which the hydraulic pressure in the brake device is decreased in response to drop of the wheel speed when the brakes are applied to cause the braking hydraulic pressure to be supplied to the brake device for each wheel and the hydraulic pressure in the brake device is increased, and in which the hydraulic pressure is increased again after recovery of the wheel speed due to the decrease in hydraulic pressure in the brake device, and the same control pattern is repeated whereby effective braking operation can be accomplished. Such a type of antilock brake control device is disclosed in Japanese Patent Application Laid-Open Publication (KOKAI) No. 60-61354 published Apr. 9, 1985.
In the case of conventional antilock brake devices of the type described above, in response to wheel-speed signals delivered from wheel speed sensors provided for respective wheels, control is carried out for decreasing and reincreasing the braking hydraulic pressure. In practice, the hydraulic brake line system is formed as a dual brake line system in which two pairs of diagonally opposing wheels are connected to two independent brake lines, respectively, so that even in case of failure of one brake line, an emergency braking force can be produced by the other brake line. Generally, in this case, the lower wheel speed of the speeds of the two wheels connected to the same brake line is selected, and in response to the selected lower wheel speed, the braking hydraulic pressure control in that brake line is carried out so that all the wheels are prevented from locking.
However, in the case of J-turn (which is the state when a vehicle enters a curve at a speed in excess of a predetermined value and the driver makes a sharp turn and which is a state well known to those skilled in the art pertaining to the subject matter of the present invention), when the antilock brake device operates in the manner described above, the difference in wheel speed between the inner and outer wheels becomes large and the lateral acceleration is produced so that the inner wheels tend to rise up and the reaction forces from the surface of the road acting on the inner wheels are consequently decreased. As a result, the wheel speeds of the inner wheels are considerably decreased, and consequently the control for decreasing and increasing the braking hydraulic pressure is carried out mainly on the basis of the wheel speeds of the inner wheels. Then, since the reaction forces from the surface of the road acting on the inner wheels are low as described above, the wheel speeds are abruptly decreased in response to slight increase in the hydraulic pressure, and consequently the wheel are brought to a state in which the wheels are almost locked. Even when the hydraulic pressure is decreased under such conditions, it takes a long time for the wheel speeds recover, and the "no braking" mode (the hydraulic pressure is decreased) becomes longer. As a consequence, the braking force becomes insufficient as a whole, so that there arises the problem that the distance through which the vehicle travels after braking becomes longer than expected in an extremely dangerous state in which the brakes are abruptly applied in J-turn state, which seriously inhibits the safety drive.